Nitrogen-Doped Ordered Mesoporous Carbons Supported Co 3 O 4 Composite as a Bifunctional Oxygen Electrode Catalyst

It is increasingly useful to develop bifunctional catalysts for oxygen reduction and oxygen evolution reaction (ORR and OER) for fuel cells, metal-air rechargeable batteries, and unitized regenerative cells. Here, based on the excellent conductivity and stability of ordered mesoporous carbons, and the best ORR and OER activity of Co3O4, the composite Co3O4/N-HNMK-3 was designed and manufactured by means of a solvothermal method, using ordered N-doped mesoporous carbon (N-HNMK-3) as substrate, and then the bifunctional electrocatalytic performance corresponding to ORR, OER in alkaline media was carefully investigated. The results showed that Co3O4/N-HNMK-3 composite, a non-precious metal centered electrocatalyst, displayed excellent ORR performance (activity, selectivity, and stability) close to that of commercial 20 wt.% Pt/C and a promising OER activity near 20 wt.% RuO2/C. The outstanding bifunctional activities of Co3O4/N-HNMK-3 was assessed with the lowest 4E value of 0.86 V (EOER,10 mA cm−2 -EORR,−3 mA cm−2 ) with respect to the two commercial precious metal-based electrocatalysts.


Introduction
Hydrogen is deemed as the best way for renewable sources energy-storage to replace petroleum-based energy resources because of its high calorific value, abundant reserve, and near zero release.Yet, both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) individually play crucial roles in fuel cells for the practical application of hydrogen, and water electrolysis for the high-efficiency production of high-purity hydrogen [1][2][3][4].Recently, unitized regenerative cells (URCs), integrating water electrolysis and fuel cell in one setup, have attracted increasing attention due to potential applications in aviation, submarines and transportation as a mobile power source.This issue remains key to exploring high performance and low cost.In terms of electrocatalytic activity, Pt/C and Ru-/Ir oxides are generally considered to be the most advanced electrochemical oxygen catalysts for the ORR and OER processes, respectively.However, high cost, low methanol tolerance, and poor stability continue to hamper practical marketing development [5].
Therefore, it remains a big challenge to develop promising bifunctional oxygen electrodes for the development of URCs.
Among non-precious metal electrocatalysts being developed such as cobalt-based chalcogenides, CoSe 2 , CoS, and Co 3 O 4 , show outstanding performance toward oxygen electrode reactions including activity, selectivity, and durability in alkaline media [6][7][8][9].However, cobalt-based chalcogenide electrocatalysts have a low electrical conductivity and per se a negative effect on the performance of the electrocatalyst.On the other hand, ordered mesoporous carbon as a conductive substrate has attracted more attention because of large surface, high conductivity and long durability.[10][11][12][13].In addition, the doping heteroatom, such as nitrogen [14,15], phosphor [16,17], sulfur [18] and boron [19], can further improve electrocatalytic performance towards oxygen electrode reactions due to new active species.In a previous report, we developed N-doped ordered mesoporous carbons (CMK-3) with high ORR activity, long stability, and outstanding methanol tolerance in 0.1 M KOH [20].Therefore, it is possible to explore high-performance, low-cost electrocatalysts using N-doped ordered mesoporous carbon as a substrate to support cobalt-based chalcogenides.
Here, we manufactured the N-doped ordered mesoporous carbon substrate (N-HNMK-3) for Co 3 O 4 composite (Co 3 O 4 /N-HNMK-3) by means of a solvothermal route using the home-made ordered mesoporous carbon as carbon source, melamine as a nitrogen source, cobalt acetate as a cobalt source, and urea as the precipitating agent.Figure 1 describes the corresponding synthesis process.In addition, we carefully examined the bifunctional electrocatalytic performance of ORR and OER processes in alkaline media using the rotating disc electrode technique.The compound, based on non-precious metals, shows a promising electrocatalytic performance.
Surfaces 2019, 2 FOR PEER REVIEW 2 Among non-precious metal electrocatalysts being developed such as cobalt-based chalcogenides, CoSe2, CoS, and Co3O4, show outstanding performance toward oxygen electrode reactions including activity, selectivity, and durability in alkaline media [6][7][8][9].However, cobalt-based chalcogenide electrocatalysts have a low electrical conductivity and per se a negative effect on the performance of the electrocatalyst.On the other hand, ordered mesoporous carbon as a conductive substrate has attracted more attention because of large surface, high conductivity and long durability.[10][11][12][13].In addition, the doping heteroatom, such as nitrogen [14,15], phosphor [16,17], sulfur [18] and boron [19], can further improve electrocatalytic performance towards oxygen electrode reactions due to new active species.In a previous report, we developed N-doped ordered mesoporous carbons (CMK-3) with high ORR activity, long stability, and outstanding methanol tolerance in 0.1 M KOH [20].Therefore, it is possible to explore high-performance, low-cost electrocatalysts using N-doped ordered mesoporous carbon as a substrate to support cobalt-based chalcogenides.
Here, we manufactured the N-doped ordered mesoporous carbon substrate (N-HNMK-3) for Co3O4 composite (Co3O4/N-HNMK-3) by means of a solvothermal route using the home-made ordered mesoporous carbon as carbon source, melamine as a nitrogen source, cobalt acetate as a cobalt source, and urea as the precipitating agent.Figure 1 describes the corresponding synthesis process.In addition, we carefully examined the bifunctional electrocatalytic performance of ORR and OER processes in alkaline media using the rotating disc electrode technique.The compound, based on non-precious metals, shows a promising electrocatalytic performance.

Synthesis of Nitrogen-doped Ordered Mesoporous Carbon (N-HNMK-3)
Nitrogen-doped ordered mesoporous carbon was fabricated by post-treated nitrogen doping method using ordered mesoporous carbon (CMK-3, surface area, 1247.5 m 2 /g; pore volume, 1.5 mL/g) as carbon source, and melamine as nitrogen source.To obtain HNMK-3, CMK-3 (0.5002 g) was dispersed in 6.0 M HNO3 (50.0 mL) under vigorous stirring at 70 °C for 3 h, and the resulting suspension was cleaned to neutral pH using deionized water.Thereafter, 0.5000 g HNMK-3 and 2.5000 g melamine were then mixed during grinding to form the homogeneous mixture.Finally, the obtained mixture was calcined in a nitrogen atmosphere at 900 °C for 4 h to produce nitrogen-doped ordered mesoporous carbon (N-HNMK-3).

Synthesis of Nitrogen-Doped Ordered Mesoporous Carbon (N-HNMK-3)
Nitrogen-doped ordered mesoporous carbon was fabricated by post-treated nitrogen doping method using ordered mesoporous carbon (CMK-3, surface area, 1247.5 m 2 /g; pore volume, 1.5 mL/g) as carbon source, and melamine as nitrogen source.To obtain HNMK-3, CMK-3 (0.5002 g) was dispersed in 6.0 M HNO 3 (50.0mL) under vigorous stirring at 70 • C for 3 h, and the resulting suspension was cleaned to neutral pH using deionized water.Thereafter, 0.5000 g HNMK-3 and 2.5000 g melamine were then mixed during grinding to form the homogeneous mixture.Finally, the obtained mixture was calcined in a nitrogen atmosphere at 900 • C for 4 h to produce nitrogen-doped ordered mesoporous carbon (N-HNMK-3).

Synthesis of Co 3 O 4 /N-HNMK-3 Electrocatalyst
Co 3 O 4 /N-HNMK-3 electrocatalyst was carefully generated via the solvothermal route using N-HNMK-3 as conductive support; Co(CH 3 COO) 2 as cobalt source, and CO(NH 2 ) 2 as the precipitating agent.Typically, 0.0175 g of the as-prepared N-HNMK-3 was dispersed in ethanol (40.0 mL) by ultrasound for 2h to form the suspension A. Simultaneously, 1.50 g Co(Ac) 2 (6.0 mmol) was added in deionized water (40.0 mL) and ultra-sonicated for 20 min to form solution B. Then suspension A and solution B were mixed and ultrasonicated again to produce another suspension.At this point 0.72 g CO(NH) 2 was added under vigorous stirring at 80 • C for 24 h to produce a suspension, which was heated at 150 • C in a Teflon-lined stainless-steel autoclave for 3 h.The cooled product was centrifuged and washed with deionized water and ethanol three cycles, and dried at 60 • C overnight.The collected final product was Co 3 O 4 /N-HNMK-3.In addition, Co 3 O 4 , as a reference, was prepared in the same procedure without N-HNMK-3.

Characterization
The materials phase was characterized by Rigaku UItima III XRD (Rigaku Corporation, Tokyo, Japan) with Cu K α radiation (λ = 0.154 nm) at 5 min −1 per 2θ.The morphologies were analyzed by Zeiss Supra 55 SEM(Carl Zeiss Jena, Oberkochen, German), a JEOL JEM-2012 TEM (JEOL, Tokyo, Japan), and HRTEM (JEOL, Tokyo, Japan) with a line resolution of 0.19 nm was used.Raman spectra were examined via a Nanofinder 3.0 Raman spectrometer with a He-Ne laser beam of 532 nm.Binding energies of chemical species were collected by XPS using VG ESCALAB 2201 XL spectrometer (Thermo VG Scientific, West Sussex, England).

Electrochemical Measurements
All electrochemical tests were performed in a standard three-electrode system on the Chen Hua electrochemical workstation (CH Instruments Ins., Shanghai, China) at 25 • C. Here, a saturated calomel electrode and a Pt-wire were employed as the reference and counter electrodes, respectively.In this work, all potentials were referred to a reversible hydrogen electrode (RHE): E RHE = E SCE + 0.99 V (0.1 M KOH); and E RHE = E SCE + 1.06 V (1.0 M KOH).
Prior to catalyst deposition, the GC disk was polished with γ-alumina (5A), and successively ultra-sonically treated in water, and then ethanol for one min in each solvent.The ink was prepared by mixing the catalyst powder (6.7 mg), Nafion (5 wt.%) (40 µL), ethanol (300 µL), deionized water (1160 µL) and ultra-sonicated for 40 min.4.2 µL of the ink was deposited onto the GC disk (4.0 mm, diameter) with a catalyst mass loading of 150 µg cm −2 .The electrolyte was degassed with argon and oxygen for 30 min before cyclic voltammetry (CV) tests.Also, two benchmarks: 20 wt.% Pt/C (Johnson Matthey) and RuO 2 /C were prepared as references with a corresponding loading of 60 µg cm −2 for Pt, and RuO 2 .
In the ORR test, the LSV curves were recorded in O 2 -saturated 0.1 M KOH by scanning the disc potential vs. RHE from 0.95 V to 0.15 V at 5 mVs −1 with the electrode rotated at 2500, 2025, 1600,1225, 900, 625, and 400 rpm, and analysis performed using the Koutecky-Levich (K-L) equation: where: (2) j, j k , j d are the corresponding measured, kinetic and diffusion limiting current densities, respectively [5]; C 0 and D 0 were the saturated concentration, and the diffusion coefficient of O 2 (1.14 × 10 −6 mol cm −3 and 1.73 × 10 −5 cm s −1 ) in 0.1 M KOH, v was the kinetic viscosity coefficient (0.01 cm 2 s −1 ).
In the RRDE test, the HO 2 − % and the number of electrons transferred(n) was calculated by the following two equations: where I D , I R and N is disk current, ring current and collection efficiency (0.424), respectively.For the OER tests, measurements were carried out in O 2 -saturated 1.0 M KOH with a rotating speed of 1600 rpm at a scan rate of 5 mV s −1 from 1.0 V to 1.65V vs. RHE.j, jk, jd are the corresponding measured, kinetic and diffusion limiting current densities, respectively [5]; C0 and D0 were the saturated concentration, and the diffusion coefficient of O2 (1.14 × 10 −6 mol cm −3 and 1.73 × 10 −5 cm s −1 ) in 0.1 M KOH, v was the kinetic viscosity coefficient (0.01 cm 2 s −1 ).

Structure, Morphology, and Chemical Composition
In the RRDE test, the HO2 − % and the number of electrons transferred(n) was calculated by the following two equations: where ID, IR and N is disk current, ring current and collection efficiency (0.424), respectively.For the OER tests, measurements were carried out in O2-saturated 1.0 M KOH with a rotating speed of 1600 rpm at a scan rate of 5 mV s −1 from 1.0 V to 1.65V vs. RHE.

Structure, Morphology, and Chemical Composition
Figure 2 shows the powder PXRD patterns of N-HNMK-3, Co3O4, and Co3O4/N-HNMK-3.Compared to CMK-3, the peak of N-HNMK-3 samples at 23.8°/2θ shifts to a lower angle with a higher degree of graphitization, which could be the result of a change in electron distribution following the incorporation of the heteroatom on the surface of carbon materials [21].Based on the Bragg equation, a series of Bragg reflection peaks in Co3O4 and Co3O4/N-HNMK-3 samples correspond to (111), ( 220), (311), ( 400), (511), and (440) crystal faces, that match well with the cubic spinel phase Co3O4 (ICDD PDF card No. 42-1467).This means that the urea-assisted solvothermal method is an available way to prepare the Co3O4/N-HNMK-3 composite.In addition, Figure 4 shows the Raman spectra of N-HNMK-3, Co3O4, and Co3O4/N-HNMK-3 samples as well as the corresponding fitting profiles of N-HNMK-3 and Co3O4/N-HNMK-3.One observes two characteristic bands for N-HNMK-3, and Co3O4/N-HNMK-3 centered at 1358 (D peak) and 1590 (G peak) cm −1 .The former peak (D peak) refers to sp 3 -like C atoms defect sites, whereas the latter (G peak) refers to sp 2 C=C stretching mode [22].Remarkably, other peaks were detected at 192, 468, 516 and 677 cm −1 , assessing the presence of Co3O4 [23].The intensity ratio of two bands (ID/IG) illustrates the disordered degree of carbonization.A ratio of 2.96 for N-HNMK-3, and 2.78 for Co3O4/N-HNMK-3 suggest that the incorporation of Co3O4 decreases the disordered degree, increasing the conductivity of the substrate.In addition, Figure 4 shows the Raman spectra of N-HNMK-3, Co3O4, and Co3O4/N-HNMK-3 samples as well as the corresponding fitting profiles of N-HNMK-3 and Co3O4/N-HNMK-3.One observes two characteristic bands for N-HNMK-3, and Co3O4/N-HNMK-3 centered at 1358 (D peak) and 1590 (G peak) cm −1 .The former peak (D peak) refers to sp 3 -like C atoms defect sites, whereas the latter (G peak) refers to sp 2 C=C stretching mode [22].Remarkably, other peaks were detected at 192, 468, 516 and 677 cm −1 , assessing the presence of Co3O4 [23].The intensity ratio of two bands (ID/IG) illustrates the disordered degree of carbonization.A ratio of 2.96 for N-HNMK-3, and 2.78 for Co3O4/N-HNMK-3 suggest that the incorporation of Co3O4 decreases the disordered degree, increasing the conductivity of the substrate.

Electrocatalytic ORR Performance
The ORR performance of Co3O4/N-HNMK-3 composite was determined by RDE technique, and compared to two commercial reference materials: Pt/C and RuO2/C.Figure 6 displays the activity towards ORR in 0.1 M KOH electrolyte.Specifically, Figure 6a, b show the ORR activity difference among five samples: N-HMNK-3, physical mixture of N-HMNK-3 and Co3O4, Co3O4/N-HMNK-3 composite, Pt/C, and RuO2/C.Herein, it is observed that the Co3O4/N-HMNK-3 composite has a much higher ORR activity compared to N-HMNK-3, the physical mixture, and RuO2/C, and close to Pt/C.Interestingly, the Co3O4/N-HNMK-3 composite shows much better ORR activity than the mechanical

Electrocatalytic ORR Performance
The ORR performance of Co 3 O 4 /N-HNMK-3 composite was determined by RDE technique, and compared to two commercial reference materials: Pt/C and RuO 2 /C. Figure 6  mixture (Co3O4 +N-HNMK-3).This difference in the ORR activity results from the possible synergistic effect between two compositions, and from the high dispersion of Co3O4 on the N-HNMK-3 substrate.
In addition, cyclic voltammetry measurement, shown in Figure 6c, was conducted to gain deeper insight into the ORR process.In the O2-saturated electrolyte there is a cathodic peak centered at 0.75 V, which is absent in the Ar-saturated electrolyte.The onset potential of Co3O4/N-HMNK was detected at 0.90 V vs. RHE.The ORR hydrodynamics on Co3O4/N-HNMK-3 between 400 rpm to 2500 rpm is shown in Figure 6d with well-defined sigmoidal curves.The electron transfers number (n) was estimated through the slopes of fitted K-L plots, see the inset in Figure 6d.Within the applied electrode potential from 0.2 to 0.5 V, the estimated transfer number (n) per O2 molecule was ca.3.94, suggesting a 4-electron transfer pathway.Meanwhile, we used the RRDE to calculate the hydrogen peroxide and electron transfer number in Figure S1 (See Supporting Information).The low hydrogen peroxide below 15% and transfer number beyond ca.3.75 in the range of 0.1-0.75V suggested a close 4-electron transfer pathway.In addition to activity, the selectivity (tolerance to small organics) and the stability are two other key parameters for a high performance electrocatalyst.Figure 7a depicts the electrocatalytic selectivity in the presence and the absence of 3.0 M methanol in 0.1 M KOH electrolyte.After the addition of 3 M methanol, no significant change was observed in the polarization curves in Ar-and/or O2-saturated electrolyte, see the inset in Figure 7a.In O2-saturated electrolyte Co3O4/N-HNMK-3 showed a complete tolerance to methanol.In contrast, as it is well established, Pt/C has no selectivity towards the ORR in presence of methanol, since this material develops a mixed potential due to methanol oxidation and oxygen reduction.Furthermore, the stability test, as demonstrated by chronoamperometry, Figure 7b, was conducted at 0.70 V/RHE, and revealed that, after 18000 s, the In addition to activity, the selectivity (tolerance to small organics) and the stability are two other key parameters for a high performance electrocatalyst.Figure 7a depicts the electrocatalytic selectivity in the presence and the absence of 3.0 M methanol in 0.1 M KOH electrolyte.After the addition of 3 M methanol, no significant change was observed in the polarization curves in Ar-and/or O 2 -saturated electrolyte, see the inset in Figure 7a.In O 2 -saturated electrolyte Co 3 O 4 /N-HNMK-3 showed a complete tolerance to methanol.In contrast, as it is well established, Pt/C has no selectivity towards the ORR in presence of methanol, since this material develops a mixed potential due to methanol oxidation and oxygen reduction.Furthermore, the stability test, as demonstrated by chronoamperometry, Figure 7b, was conducted at 0.70 V/RHE, and revealed that, after 18000 s, the ORR activity of Co 3 O 4 /N-HNMK-3 dropped 18.2% against 23.9% on Pt/C.Meanwhile, CVs with multiple cycles in oxygen-saturated solution for ORR were shown in Figure S2 (See Supporting Information), after the accelerated aging test, the reduction potential only shifts 36 mV to a negative region for 1500 cycles.Information), after the accelerated aging test, the reduction potential only shifts 36 mV to a negative region for 1500 cycles.

Electrocatalytic OER Performance
The OER plays a crucial role in water electrolysis and metal-air batteries' systems [26].In particular, high-performance OER electrocatalysts aim at reducing energy costs to produce high purity hydrogen and promote the development of URCs. Figure 8 shows the activity of Co3O4/N-HNMK-3 compared to N-HNMK-3, physical mixture of Co3O4 + N-HNMK-3, Pt/C, and RuO2/C in 1.0 M KOH.Apart from RuO2/C, the Co3O4/N-HNMK-3 has the highest OER activity with a higher current density of 19.56 mA cm −2 at 1.65 V, lower overpotential (365 mV) at the current density of 10 mA cm −2 , a value close to the best OER electrocatalyst RuO2/C.The Tafel plots for the OER kinetics are shown in Figure 8b.The slope of 93 mV dec −1 for Co3O4/N-HNMK-3 is higher than that of 74 mV dec −1 for RuO2/C, and lower than that of the physical mixture.In addition, the OER stability test, via CV, performed on Co3O4/N-HNMK-3 is shown in Figure 8c,d.No significant variation was observed after 1500 cycles, and to some extent, Co3O4/N-HNMK-3 showed a ca.4.1% increase in the overpotential at 10 mA cm −2 after 1500 cycles.The results confirm that Co3O4/N-HNMK-3 is quite durable for the OER in 1.0 M KOH electrolyte.

Electrocatalytic OER Performance
The OER plays a crucial role in water electrolysis and metal-air batteries' systems [26].In particular, high-performance OER electrocatalysts aim at reducing energy costs to produce high purity hydrogen and promote the development of URCs. Figure 8

The Bifunctional ORR/OER Performance
The overvoltage between ORR and OER is an important parameter to evaluate the bifunctional electrocatalytic activity of the material.The corresponding data are summarized in Table 2.The ΔE value is defined as the difference in potential between the OER current density of 10.0 mA cm −2 and the ORR current density of −3.0 mA cm −2 .Among the listed catalysts [27][28][29][30][31][32] reported in the literature, Co3O4/N-HNMK-3 shows the best and promising bifunctional activity with a minimum ΔE value, which is attributed to the interaction developed between Co3O4 species onto N-doped ordered mesoporous carbons.

The Bifunctional ORR/OER Performance
The overvoltage between ORR and OER is an important parameter to evaluate the bifunctional electrocatalytic activity of the material.The corresponding data are summarized in Table 2.The ∆E value is defined as the difference in potential between the OER current density of 10.0 mA cm −2 and the ORR current density of −3.0 mA cm −2 .Among the listed catalysts [27][28][29][30][31][32] reported in the literature, Co 3 O 4 /N-HNMK-3 shows the best and promising bifunctional activity with a minimum ∆E value, which is attributed to the interaction developed between Co 3 O 4 species onto N-doped ordered mesoporous carbons.

Conclusions
In this paper, a nitrogen-doped ordered mesoporous carbons supported Co 3 O 4 composite (Co 3 O 4 /N-HNMK-3) was prepared through a solvothermal method.The Co 3 O 4 /N-HNMK-3 composite showed a much higher ORR performance, which was close to that of the commercial Pt/C in 0.1 M KOH.An excellent OER performance near commercial RuO 2 /C in 1.0 M KOH was also obtained.Interestingly, Co 3 O 4 /N-HNMK-3 has outstanding bifunctional activity with a much lower E value between E OER,10 mA cm −2 -E ORR,−3 mA cm −2 as compared to Pt/C and RuO 2 /C in alkaline medium.Undoubtedly, the synthesis method developed here to obtain the oxygen electrode based on Co 3 O 4 /N-HNMK-3, with non-precious metals will promote the practical development of fuel cells, water electrolysis and unitized regenerative cells.

Figure 2
Figure 2 shows the powder PXRD patterns of N-HNMK-3, Co 3 O 4 , and Co 3 O 4 /N-HNMK-3.Compared to CMK-3, the peak of N-HNMK-3 samples at 23.8 • /2θ shifts to a lower angle with a higher degree of graphitization, which could be the result of a change in electron distribution following the incorporation of the heteroatom on the surface of carbon materials [21].Based on the Bragg equation, a series of Bragg reflection peaks in Co 3 O 4 and Co 3 O 4 /N-HNMK-3 samples correspond to (111), (220), (311), (400), (511), and (440) crystal faces, that match well with the cubic spinel phase Co 3 O 4 (ICDD PDF card No. 42-1467).This means that the urea-assisted solvothermal method is an available way to prepare the Co 3 O 4 /N-HNMK-3 composite.

Figure 3
Figure 3 displays the morphologies and microstructures of Co 3 O 4 and Co 3 O 4 /N-HNMK-3 samples.The Co 3 O 4 sample shows different rod-shape in size, Figure 3a.In comparison, the Co 3 O 4 particles, in Co 3 O 4 /N-HNMK-3 composite, are well-dispersed and supported on N-HNMK-3 substrate, Figure 3b,c.In addition, the average particle size for Co 3 O 4 was 20-30 nm, Figure 3c.The HRTEM image of Co 3 O 4 reveals three lattice distances such as 0.286, 0.244 and 0.202 nm, which individually correspond to (220), (311) and (400) Co 3 O 4 crystal planes, Figure 3d.
displays the activity towards ORR in 0.1 M KOH electrolyte.Specifically, Figure 6a,b show the ORR activity difference among five samples: N-HMNK-3, physical mixture of N-HMNK-3 and Co 3 O 4 , Co 3 O 4 /N-HMNK-3 composite, Pt/C, and RuO 2 /C.Herein, it is observed that the Co 3 O 4 /N-HMNK-3 composite has a much higher ORR activity compared to N-HMNK-3, the physical mixture, and RuO 2 /C, and close to Pt/C.Interestingly, the Co 3 O 4 /N-HNMK-3 composite shows much better ORR activity than the mechanical mixture (Co 3 O 4 +N-HNMK-3).This difference in the ORR activity results from the possible synergistic effect between two compositions, and from the high dispersion of Co 3 O 4 on the N-HNMK-3 substrate.In addition, cyclic voltammetry measurement, shown in Figure 6c, was conducted to gain deeper insight into the ORR process.In the O 2 -saturated electrolyte there is a cathodic peak centered at 0.75 V, which is absent in the Ar-saturated electrolyte.The onset potential of Co 3 O 4 /N-HMNK was detected at 0.90 V vs. RHE.The ORR hydrodynamics on Co 3 O 4 /N-HNMK-3 between 400 rpm to 2500 rpm is shown in Figure 6d with well-defined sigmoidal curves.The electron transfers number (n) was estimated through the slopes of fitted K-L plots, see the inset in Figure 6d.Within the applied electrode potential from 0.2 to 0.5 V, the estimated transfer number (n) per O 2 molecule was ca.3.94, suggesting a 4-electron transfer pathway.Meanwhile, we used the RRDE to calculate the hydrogen peroxide and electron transfer number in Figure S1 (See Supporting Information).The low hydrogen peroxide below 15% and transfer number beyond ca.3.75 in the range of 0.1-0.75V suggested a close 4-electron transfer pathway.Surfaces 2019, 2 FOR PEER REVIEW 7
shows the activity of Co 3 O 4 /N-HNMK-3 compared to N-HNMK-3, physical mixture of Co 3 O 4 + N-HNMK-3, Pt/C, and RuO 2 /C in 1.0 M KOH.Apart from RuO 2 /C, the Co 3 O 4 /N-HNMK-3 has the highest OER activity with a higher current density of 19.56 mA cm −2 at 1.65 V, lower overpotential (365 mV) at the current density of 10 mA cm −2 , a value close to the best OER electrocatalyst RuO 2 /C.The Tafel plots for the OER kinetics are shown in Figure 8b.The slope of 93 mV dec −1 for Co 3 O 4 /N-HNMK-3 is higher than that of 74 mV dec −1 for RuO 2 /C, and lower than that of the physical mixture.In addition, the OER stability test, via CV, performed on Co 3 O 4 /N-HNMK-3 is shown in Figure 8c,d.No significant variation was observed after 1500 cycles, and to some extent, Co 3 O 4 /N-HNMK-3 showed a ca.4.1% increase in the overpotential at 10 mA cm −2 after 1500 cycles.The results confirm that Co 3 O 4 /N-HNMK-3 is quite durable for the OER in 1.0 M KOH electrolyte.

Table 2 .
The oxygen bifunctional electrocatalytic performance of catalysts prepared in this work compared with other bifunctional catalysts from the literature.

Table 2 .
The oxygen bifunctional electrocatalytic performance of catalysts prepared in this work compared with other bifunctional catalysts from the literature.